The present invention relates generally to optical transmission systems and more particularly to an optical communication network including a low noise surface emitting laser being operated in multiple transverse modes or multiple filamentation.
Optical communication systems, used to carry information from one location to another, are comprised of at a minimum, three elements: (1) a transmitter that generates a beam of light and modulates the beam with data to be transmitted, (2) a receiver that receives the beam of light and recovers data from it, and (3) a medium such as an optical fiber that carries the beam of light from the transmitter to the receiver. Light may travel through an optical medium in a single mode or in multiple modes.
Multiple mode operation has generally been understood to consist of multiple modes in one laser cavity. However, multiple mode operations can occur with filamentation due to non-uniform gain or loss. This is specially true for laser with a large transverse dimension compared with the wave length. For convenience, the terms "multiple mode" and "multimode" as used herein to describe the laser operation will include both multiple mode in a single laser cavity and multiple filamentation.
A given optical medium may be capable of supporting many modes or only a single mode. This is determined by physical dimensions and parameters of the optical medium--in the case of an optical fiber--the diameter of the fiber and the difference between the indices of refraction of the core and the cladding.
Optical communication systems using multimode fibers (MMF) are subject to degradation in its performance caused by such parameters as intermodal dispersion, and modal noise due to mode selective losses (MSL). Intermodal dispersion becomes worse as the fiber length increases. Since intermodal dispersion only affects multimode fibers, single mode fiber (SMF) systems are preferred for long distances.
FIGS. 1A and 1B plot relative intensity noise (RIN) versus frequency for an optical communication system using single mode fiber and a laser operating in single mode. The relaxation oscillation of a laser is the characteristic modulation frequency of the laser and is proportional to the square root of the photon density in the cavity. The relaxation oscillation of curve 102 is noted by point 110 on the frequency axis of FIG. 1A. The relaxation oscillation of curve 104 is noted by point 112 on the frequency axis of FIG. 1B. The peak of the relative intensity noise (RIN) occurs at the relaxation frequency and is noted by points 114 and 116 for FIGS. 1A and 1B respectively.
Comparing the lasers of FIG. 1A and FIG. 1B, the laser in FIG. 1B has a mirror reflectivity greater than the mirror reflectivity of FIG. 1A at the same current bias. (Reflectivity is defined as the geometric mean reflectivity of both mirrors unless specifically stated as the reflectivity of a specific mirror.) Since the photon density is higher for the laser in FIG. 1B compared to the laser in FIG. 1A, the relaxation oscillation of the laser in FIG. 1B is also higher than the relaxation oscillation of the laser in FIG. 1A. The article "Relative intensity noise of vertical cavity surface emitting lasers", D. M. Kuchta et. al., Appl. Phys. Lett 62, 15 Mar. 1993, pp 1194-1196, describes the relationship between relative intensity noise and mirror reflectivity for a single mode laser.
Assuming the same current densities, although the laser represented by curve 104 has a higher relaxation frequency, it is less efficient than the laser of curve 102. Typically lasers are designed for high efficiency where the ratio of power(optical)/Power(electrical) is made as large as possible. High efficiency requires designing the laser with optimized output power. Lasers are typically designed for high efficiency to minimize excess heat generation which can deteriorate laser performance.
Multimode optical fiber is used in short distance optical links (&lt;10 kilometers) due to the low cost of the components. Multiple mode fibers are preferred because the relative ease of packaging and alignment makes MMF systems considerably less expensive than single mode fiber systems. For communicating over distances of less than a few hundred meters, existing optical communication systems have used multimode fibers for the local area.
Multimode lasers typically have higher RIN than single mode lasers. FIGS. 2A and 2B show a graphical representation of relative intensity noise (RIN) versus frequency for an optical communication system using multimode fiber and a vertical cavity surface emitting laser (VCSEL) operating in multiple mode/filaments. Multimode lasers tend to exhibit multiple relaxation oscillation frequencies and have multiple RIN peaks. For example, the three curves 202, 204, 206 shown in FIG. 2A each represent the relaxation frequency for a different mode of the multimode laser. The RIN of the multimode laser shown in FIG. 2B is a composite of the sum of curves 202, 204, 206. The multimode laser's RIN is the summation of the different modal/filament RIN of the laser, the RIN for the multimode laser is typically higher than the RIN for a single mode laser. Comparing the curves of FIGS. 1A and 1B to FIG. 2B, the RIN labeled 120 in FIGS. 1A and 1B in general is less than the RIN labeled 220 in FIG. 2B for lasers with the same output power. Also, because the RIN for the multimode laser is a summation of curves, RIN tends to be a more complex waveform that is less predictable than the RIN waveform for a single mode laser.
From the foregoing it will be apparent that a low noise, high speed, and economical way to transmit data at data rates exceeding one gigabit per second by means of optical communication systems using a multimode laser to transmit data is needed.